diode to door trigger

http://en.wikipedia.org/wiki/Check_valve

A diode is an electronic check valve. 

Start with basics;  having found the door contact wire, open a second door and shut the first, if your meter shows door open still, you won't need diodes and so on for all 4 or 5 doors.  Secondly if your domelight goes out immediately on closing driver's door same again. If the above isn't true then the door contacts (called pin switches) are on separate circuits, eg to do with key sense, door open warnings on instrument panel etc. Thus you would connect to each one and then join together via diodes. With few exceptions these door contacts are negative going or the wire goes to ground on opening, thus band towards your alarm, what you've done is to prevent any false signals going back to the car and messing up processors etc.  The second adaptation is where the BCM  like on nearly all newer cars "goes to sleep" to conserve juice. These units will send a self check pulse around the car at various times often once an hour, causing your alarm to trigger, thus use blocking diodes, the famous DEI 1076. Think current flow, pos to neg, band from source, block pos, reverse band, I know it's hard to explain unless you can picture the circuit intuitively in your mind.